Optical Isomerism 

What are the different types of isomerism?

Isomers can be divided into two main categories, each with sub-categories:

1. Structural isomers - same molecular formula but a different structural formula.
   1. Chain isomers
   2. Position isomers
   3. Functional group isomers
      More information: “Types of Structural Isomerism”
      
2. Stereoisomers - same molecular formula, same structural formula, but a different three-dimensional arrangement of atoms.
   1. Cis / trans or E / Z isomers
      More information: “Types of Stereoisomerism”
      
   2. Optical isomers

What is optical isomerism?

Optical isomers have the same molecular formula and structural formula but they have a different arrangement of atoms in three-dimensional space.

Optical isomerism is shown by molecules that have a carbon atom with four different atoms (or groups of atoms) attached to it in a tetrahedral shape. This carbon atom is referred to as a chiral carbon or a chiral centre as demonstrated by 1-bromoethanol:

The overall molecule is asymmetric on the chiral carbon, i.e. it has no centre, plane or axis of symmetry, which leads to two possible arrangements which are non-superimposable mirror images. 

These two mirror-image molecules can be known as:

• Optical isomers
• R and S (optical) isomers
• Enantiomers
• Racemate / racemic mixture - if both isomers are present in equal quantities

Two optical isomers will have the same physical properties with one exception. One enantiomer can rotate plane polarised light in one direction and the other enantiomer will rotate plane polarised light in the opposite direction. This property is used to distinguish between two different optical isomers.

How to identify optical isomers and chiral centres

Optical isomers must have a chiral centre. For organic compounds, this means that there is a carbon atom with four different atoms or groups of atoms. This is commonly annotated as an asterisk (*) on structures.

Note: Any carbon atom with a double bond or two of the same atoms or groups of atoms cannot be a chiral centre.

Some example molecules that contain chiral centres include:

Alanine	Butan-2-ol	Lactic acid

Remember: In skeletal formulae, none of the hydrogen atoms attached to carbon atoms are shown. This is important because hydrogen is often the fourth atom of the chiral centre.

Tartaric acid is a more complex example because it contains more than one chiral centre:

How to draw optical isomers

Optical isomers are drawn using dot and wedge bonds to give a 3-D representation of the molecule. Start by drawing the chiral carbon and then add:

• Two standard single bonds are drawn in the plane
• One wedge bond is drawn to show that atom or group of atoms is coming out of the plane of the molecule
• One dashed bond is drawn to show that atom or group of atoms is going behind the plane of the molecule

Next, draw the mirror image of the structure.

Finally, add the four different atoms or groups of atoms.

Alanine - optical isomers	Butan-2-ol - optical isomers

	Lactic acid - optical isomers

Optical isomerism in transition metal complexes 

Tetrahedral transition metal complexes can theoretically exhibit optical isomerism if there are four different ligands. However, this is not typically seen due to the requirement of having four different ligands complexed to the central metal ion in a tetrahedral shape.

Octahedral transition metal complexes can show optical isomerism. This is most common with bidentate ligands:

An octahedral transition metal complex with two monodentate ligands (L) and two bidentate ligands will exhibit a combination of geometric / cis-trans isomerism and optical isomerism:

Cis isomer Optically active	Cis isomer Optically active	Trans isomer Optically inactive

Optical isomers in drugs

Some examples of drugs that are optical isomers are thalidomide, naproxen and ibuprofen. Thalidomide is a controversial drug that contains one chiral centre. Naproxen is a relatively common non-steroidal, anti-inflammatory drug (NSAID) that also contains a single chiral centre. They are both considered more complex examples due to the size of their molecules and the multiple-ring systems.

	Thalidomide - chiral centre

Naproxen - chiral centre	Ibuprofen - chiral centre

Like thalidomide and naproxen, ibuprofen is a more complex example but still only contains one chiral centre. One optical isomer of ibuprofen is the non-steroidal, anti-inflammatory drug while the other optical isomer has no medicinal effect. Ibuprofen is sold as a racemic mixture of the two optical isomers as the cost to separate them is not economically viable.

What keyword definitions do I need to know for optical isomers? 

Some keyword definitions you need to know are:

• Optical isomers - a compound with the same molecular formula and structural formula but they have a different arrangement of atoms in three-dimensional space
• Chiral carbon / centre - carbon atoms that are attached to four different atoms or groups of atoms in a tetrahedral arrangement.
  • Chiral carbon atoms are also known as 'stereogenic carbons' or 'asymmetrical carbon atoms'
• Non-superimposable - if two objects are non-superimposable, then it means you can tell them apart / distinguish them.

This is a quick summary of some key concepts on optical isomerism - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!